This invention has been designed to measure pressure changes occurring in reservoirs due to the actions of tidal forces, such as earth-tides. These pressure changes are on the order of a fraction of a psi occurring in the presence of several thousand psi of absolute pressure. The use of these small pressure changes is disclosed in the U.S. patent application Ser. No. 374,193, filed June 29, 1989 entitled "Analyzing a Hydrocarbon Reservoir by Determining the Response of that Reservoir to Tidal Forces," which was filed concurrently with this application and which is hereby incorporated by reference for all purposes.
Currently, reservoir pressure measurements are made through the use of various types of absolute pressure transducers installed, either temporarily or permanently, in a well communicating with the reservoir. These types of transducers do not have adequate precision to accurately measure earth-tide induced pressure changes.